I am attempting to do a multicam edit of a one hour presentation. There are three video camera tracks - a Canon C100, a Sony FS-100 both at 1920x1080p and a GoPro Here 3 Black at 4k 15fps. I am getting a severely lagging preview. It is virtually unusably slow. No doubt the 4k, 15fps from the GoPro is cause. I am looking for suggestions

I am not the best person to answer this since I use Excalibur for multicam and have since it came out some years ago BUT....I almost never play thru the timeline when I do multicam edits. I scrub thru it. I've done edits with up to 7 cameras and do 3 camera edits routinely by scrubbing. For me it just makes things easier.
Just a thought!

I would agree it is prob the gopro causing the most stress. Why not render a Cineform/ Sony mxf or whatever your preferred intermediate is, of the gopro file at the same rez/framerate as your other cams (30?).
Do your edit. and pan cropping you prob are doing with your 4k file, then replace it back before render.
should work a treat.

Thanks for the input guys. Unfortunately I can't do a scrub edit on this job. It's a 1+ hour presentation. One camera is on the speaker all the time. The second camera shot an alternate angle of the speaker and also picks up some audience reaction shots. The GoPro is locked down wide on the audience and then cropped for reaction shots. I need to play preview so I can tell when to change cameras. This is especially true when questions and comments are made from the audience.

Gerald, are you describing a manual proxy? That is after I make the cuts, lay the full res 4k GoPro footage down on a lower track, sync it and delete the low res proxy clips manually and drop in the full res footage in each spot and then do the crop?

No Jim,
After your footage is synced, select your whole gopro track and paste into a new instance of vegas. Make sure to put an empty event at the start if it does not start at 0 on the timeline.
Save your single gopro project with a name... gopro maybe :)
From windows explorer drag the .veg of the gopro project back into you multicam project.
Check that it is identical in sync with your original gopro track.
When all is confirmed good, delete your original gopro track.
Now, open up your single gopro project and render to a new track your intermediate, maybe a Sony MXF 1080p 30fps file. Now mute your 4k track video and audio.
When you go back to your multicam project it will update its sfap nested file and you will be multi camming with the lower res file from your nested project.
Before rendering your multi, go back into your single gopro project and unmute your 4k file and mute or delete (I would just mute) your intermediate.